if you end up coming minneapolis way, i suggest going north for your few extra days... in august, the weather can be hot and dry in the southern half of wisc....and rather boring....
that said, i'd say forget devil's lake (in october or in winter, well, that's a different story--it's beautiful)..... if you go north, straight up US-51 to lake superior, you will be wowed by what you'll see....
my top recommendation for your trip would be porcupine mountains state park (yep, we actually do have mountains in the midwest)... it'll be about 5 hours from both the dells and MPL, but WELL WORTH IT!!!!
ANOTHER option would be to visit the apostle islands national park in nw wisc... bayfield is a quaint town along the shores of lake superior... it's about 2-3 hours from MPL....
if you come via chicago, you could spend a few days in door county, wisc, along the shore of lake michigan.... it's about a 3-4 hr drive from chicago and the dells..... you can explore several state and county parks in the area...
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michigan.org/property/…
http://www.nps.gov/piro/index.htm
http://www.munising.org/pictured_rocks.php
http://www.nps.gov/apis/index.htm
http://www.doorcounty.com/
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but all things considered, whether you come via MPL or ORD, the most nature and scenery bang for the buck will be upper michigan, along the lake superior shoreline....
end of story.